This invention relates to orientation of a space vehicle and more particularly to a magnetic orientation and damping device for space vehicles.
In some application for space vehicles, such as a communications satellite, it is desirable to control a reference direction on the satellite to line up with the earth's magnetic field. This can be accomplished by carrying a bar magnet on the satellite whose axis represents the desired reference direction. The torque Q, in dyne centimeters (cm.), exerted on the bar magnet can be represented by the vector relationship: EQU Q = M .times. H (1)
where M is the magnetic moment of the bar magnet, in pole - cm., and H is the intensity of the earth's magnetic field in oersteds.
If the bar magnet is rigidly attached to the satellite, and the rotation rate of the satellite has been reduced below some critical value, then the bar magnet will oscillate about the earth's magnetic field direction with a period T.sub.o, given by ##EQU1## where I is the moment of inertia of the satellite about the axis of interest, in grams cm..sup.2. If no damping means is provided, these oscillations will continue indefinitely with no decrease in amplitude.
In a prior satellite orientation control system, the above principles were employed with the addition thereto of a damping means. A bar magnet with a magnetic moment of 9000 pole-cm. was used, and damping was provided by means of hysteresis rods which generated losses as a result of the changing component of the earth's magnetic field in the rods.
A disadvantage of this prior arrangement was that the amount of damping torque generated was extremely small, so that the decay time constant was in the order of 5.5 hours, although the natural period T.sub.o was only a few minutes.